We have developed a new classification of human pituitary adenomas, based on immunocytochemistry and electron microscopy. From further study of additional tumors we expect to establish the incidence, clinical and laboratory characteristics and response to treatment of these separate tumor types. This new functional classification should be of help to pathologists, endocrinologists, neurosurgeons and others dealing with human pituitary tumors. We have been asked to write a chapter describing our experience with this new classification for the Armed Forces Institute of Pathology's revised fascicle on pituitary tumors.